To the end of improving the handling and the riding comfort of a vehicle, it has been proposed to interconnect the oil chambers of the shock absorbers of the suspension devices for different wheels of the vehicle. According to this technology, when one of the wheels of the vehicle rides over a bump in the road surface (compression) the working fluid in the oil chamber of the shock absorber connected to this wheel is conveyed to the oil chambers of the shock absorbers connected to the rest of the wheels of the vehicle, and when one of the wheels of the vehicle drops into a depression in the road surface (extension) the working fluid in the oil chambers of the shock absorbers connected to the other wheels is conveyed to the oil chamber of the shock absorber connected to this wheel, with the result that the horizontal attitude of the vehicle is maintained in spite of the bumps and depressions in the road surface and the riding comfort of the vehicle is improved.
However, when the vehicle accelerates, decelerates or makes a turn, simply interconnecting the oil chambers of the shock absorbers of the different wheels of the vehicle would amplify the pitching and rolling motions of the vehicle. To eliminate this problem, Japanese patent laid-open publication No. 58-124312 proposes the use of throttle valves in the conduits which interconnect the oil chambers of the shock absorbers of different wheels, for controlling and shutting off the communication between them. This proposal is disadvantageous in that the necessary conduit system involves high assembling cost and the spaces for mounting the throttle valves may not be readily available in an already cramped modern automobiles.
It is also preferable if the damping factors of the shock absorbers can be adjusted according to the desire of the driver and the road condition, and various shock absorbers with adjustable damping coefficients have indeed been proposed. It will be particularly advantageous if shock absorbers with variable damping coefficients can be combined with a compound suspension system. However, a compound suspension system combining shock absorbers with variable damping coefficients have hitherto not been made available presumably due to the increased complexity of the internal structure of the shock absorbers and the conduit system and the increased assembly cost resulting from this complexity.